Re: PATCH: 0/6 [2nd try]: Add AVX support

H.J. Lu
2010-03-07T17:39:52Z

On Sun, Mar 7, 2010 at 9:04 AM, H.J. Lu <hjl.tools@gmail.com> wrote:
> On Sun, Mar 7, 2010 at 8:40 AM, H.J. Lu <hjl.tools@gmail.com> wrote:
>> On Sun, Mar 7, 2010 at 8:31 AM, H.J. Lu <hjl.tools@gmail.com> wrote:
>>> On Sun, Mar 7, 2010 at 6:37 AM, H.J. Lu <hjl.tools@gmail.com> wrote:
>>>> On Sun, Mar 7, 2010 at 6:16 AM, Mark Kettenis <mark.kettenis@xs4all.nl> wrote:
>>>>>> Date: Sat, 6 Mar 2010 14:16:34 -0800
>>>>>> From: "H.J. Lu" <hongjiu.lu@intel.com>
>>>>>>
>>>>>> AVX registers are saved and restored via the XSAVE extended state. The
>>>>>> extended control register 0 (the XFEATURE_ENABLED_MASK register), XCR0,
>>>>>> is used to determine which states, x87, SSE, AVX, ... are supported
>>>>>> in the XSAVE extended state.  XCR0 can be read with the new "xgetbv"
>>>>>> instruction.  The xstate_bv field at byte offset 512 in the XSAVE
>>>>>> extended state indicates what states the current process is in. If
>>>>>> the feature bit is cleared, the corresponding registers should be read as
>>>>>> 0. If we update a register, we should set the corresponding feature
>>>>>> bit in the xstate_bv field.
>>>>>>
>>>>>> We added PTRACE_GETREGSET and PTRACE_SETREGSET to Linux kernel to
>>>>>> fetch and store AVX registers with ptrace. Linux kernel also stores
>>>>>> XCR0 at the first 8 bytes of the software usable bytes, starting at
>>>>>> byte offset 464.
>>>>>>
>>>>>> There are total 6 patches to add AVX support for Linux.  They support:
>>>>>>
>>>>>> 1. The upper 128bit YMM registers are added for AVX support. The upper
>>>>>> 128bit YMM registers are hidden from users. Gdb combines XMM register,
>>>>>> %xmmX, with 128bit YMM register, %ymmXh, and present the whole 256bit
>>>>>> YMM register, %ymmX, as pseudo register to users.
>>>>>> 2. Backward compatible. If AVX isn't supported, SSE will be used.
>>>>>> 3. Forward compatible. If new state beyond AVX is supported in
>>>>>> the XSAVE extended state, only AVX state will be used.
>>>>>> 4. Remote gdb protocol extension. GDB will send "x86=xml" in qSupported
>>>>>> request packet to indicate that GDB supports x86 XML target desciption.
>>>>>> The gdb stub will send x86 XML target desciption if it sees "x86=xml"
>>>>>> in qSupported request packet.
>>>>>>
>>>>>> One advantage of this approach is YMM registers are actually stored as
>>>>>> XMM registers and upper YMM registers in the XSAVE extended state.  It
>>>>>> is easy and natural to access them as %xmmX and %ymmXh internally.  We
>>>>>> just need to hide %ymmXh from users.
>>>>>>
>>>>>> To support AVX on other OSes, the following changes are needed:
>>>>>>
>>>>>> 1. Kernel support to get/set the XSAVE extended state.
>>>>>> 2. Handle 8/16 upper YMM registers.
>>>>>> 3. Provide target to_read_description to return SSE or AVX target
>>>>>> description.
>>>>>> 4. Update gdbarch_core_read_description to return SSE or AVX target
>>>>>> description based on contents of core dump.
>>>>>
>>>>> Wait; there is something important missing here.  How are the new %ymm
>>>>> registers referred to in debug info?  The AMD64 SysV psABI defines the
>>>>> DWARF register Number Mapping, but the 0.99.4 draft copy I have
>>>>> doesn't define any mappings for the %ymm registers.  What mapping does
>>>>> GCC use?
>>>>>
>>>>
>>>> In gcc, XMM and YMM registers have the same register number. They map
>>>> to be the same DWARF register with different sizes.  Since XMM and YMM
>>>> registers are caller-saved, they don't appear in unwind info. So, the same
>>>> DWARF register with different sizes for XMM/YMM registers isn't a problem.
>>>>
>>>>
>>>
>>> Yes, there is a problem. amd64_dwarf_reg_to_regnum needs to map 256bit
>>> register to YMM. How do other arches solve this?
>>>
>>
>> My first approach works here since XMM and YMM register have the same
>> register numbers.  We can solve it with 2 alternatives:
>>
>> 1. Give a different DWARF register number for YMM register,
>> which is an incompatible ABI change.
>> 2. Implement YMM registers as a super set of XMM registers, which
>> is my first approach.
>>
>> Thanks Mark for pointing out this issue.
>
> Or I can provide i386_value_from_register.
>

It doesn't work on x86 since i386_value_from_register will change
regum. Can we change

typedef struct value * (gdbarch_value_from_register_ftype) (struct
type *type, int regnum, struct frame_info *frame);

to

typedef struct value * (gdbarch_value_from_register_ftype) (struct
type *type, int *regnum, struct frame_info *frame);

to support updating regnum?

Thanks.

--
H.J.